The invention relates to rotational molding apparatus, and more particularly, to apparatus for rotationally molding a kayak and a like article as disclosed in U.S. Pat. No. 4,247,279. In this arrangement, a kayak mold is carried by a carriage. The carriage moves into and out of an oven. The mold is rotated in the heated oven about mutual orthogonal pitch and roll axes. The oven is required to be quite large to accommodate the rotation of the mold about the pitch axis. Efficiency is lost by heating the space.
Rotation of the mold about the pitch and roll axes serves to distribute the plastic material, typically in a powdered form, inside the mold. The distributed plastic material is heated to a molten state and distributed by the rotational motions to provide a desired skin thickness pattern for the hull. The mold has a long axis of curvature lengthwise. Motion about the pitch axis is limited to a pure rotational motion. Distribution of the plastic material over the longitudinal curve of the mold by pure rotational motion may be difficult to control, at times. The resin material tends to hold at one end and then pass rapidly over the center. The desired thickness build up at the center may not be provided. The material may be distributed in a sloshing type action if the rotation is too rapid. It is desirable to have more control over the distribution of the plastic material in the hull than can be provided by pure rotational movement of the mold in the pitch direction. U.S. Pat. No. 3,822,980 discloses another rotational molding machine in which an elongated mold is inserted into an oven. In this case, the mold is rotated about a roll axis while the entire oven is rotated about an axis which coincides with the pitch axis of the mold. The mold rotates about the pitch axis in pure rotational motion.
Accordingly, an important object of the present invention is to provide rotational molding apparatus with a heated oven having increased heat efficiency.
Another important object of the present invention is to provide a rotational molding apparatus which will rotate a mold in two degrees of freedom while requiring minimum heated oven space.
Still another important object of the present invention is to provide a rotational molding apparatus wherein increased control may be had over the distribution of plastic material in the mold.
Another object of the invention is to provide rotational molding apparatus having a mold which can be rotated about a roll axis while being moved in a pendular motion about a pivot axis lying in a plane parallel to but displaced from the plane of a roll axis.
Another object of the invention is to provide rotational molding apparatus in which a mold is rotated about a roll axis while being swung in a long arc in a pendular motion to provide for a more even and controlled distribution of plastic material in the mold.
Another important object of the invention is to provide rotational molding apparatus having a mold which rotates about a roll axis while inside an oven which is pivoted about a pivot axis so that the amount of interior space needed to accommodate the mold is minimized.